Evaporators may be used in many different applications, including, for example, a heat exchanger. In a heat exchanger application, a working fluid may be introduced into an evaporator shell in thermal communication with one or more thermal elements, which causes the working fluid to evaporate. In flooded-type and kettle-type evaporators, the thermal elements are completely or partially immersed in the working fluid. This arrangement is relatively inefficient, because the thermal elements must transfer heat to a relatively large volume of working fluid. Falling film-type evaporators, on the other hand, distribute the working fluid above the thermal elements and allow the working fluid to cascade over the thermal elements without inundating the thermal elements. As a result, falling film-type evaporators may be more efficient than flooded-type and kettle-type evaporators in small scale applications, because it is relatively easy to keep the entire surface areas of the thermal elements wetted with the working fluid. In larger scale applications, however, the heating elements must be larger and/or more numerous, which makes it more difficult to keep the surface areas of all the thermal elements wetted with the working fluid using conventional falling film arrangements. Instead, many portions the thermal elements in these larger scale applications tend to dry out, making the evaporator less efficient.
One example of a larger scale heat exchanger may be utilized in an Ocean Thermal Energy Conversion (OTEC) application. In a conventional OTEC application, an evaporator may have a shell and tube design layout in which a working fluid having a low boiling point, such as ammonia for example, is introduced. Tubes passing through the interior of the evaporator carry relatively warm surface-level sea water, causing the ammonia in contact with the tubes to boil off and power a turbogenerator inline with the evaporator. However, a large-scale conventional falling film evaporator for an OTEC application may have a diameter in excess of three meters, which can result in many of the tubes near the bottom of the evaporator drying out. Thus, there is a need in the art for a large-scale falling-film type evaporator having improved efficiency.